N2 cleavage by silylene and formation of H2Si(μ-N)2SiH2

Fixation and functionalisation of N2 by main-group elements has remained scarce. Herein, we report a fixation and cleavage of the N ≡ N triple bond achieved in a dinitrogen (N2) matrix by the reaction of hydrogen and laser-ablated silicon atoms. The four-membered heterocycle H2Si(μ-N)2SiH2, the H2SiNN(H2) and HNSiNH complexes are characterized by infrared spectroscopy in conjunction with quantum-chemical calculations. The synergistic interaction of the two SiH2 moieties with N2 results in the formation of final product H2Si(μ-N)2SiH2, and theoretical calculations reveal the donation of electron density of Si to π* antibonding orbitals and the removal of electron density from the π bonding orbitals of N2, leading to cleave the non-polar and strong NN triple bond.

Page8 line 173: The N-N bond length is between 1.8-1.9 A. It should be noted that there is a minor bonding interaction between two nitrogen atoms under this value.From the perspective of dinitrogen activation, in this case, the N-N bond is fully cleaved and each N part should be assigned as N3-.In the EDA-NOCV analysis, however, the authors have chosen two neutral fragments for comparison.The rationality of this choice should be discussed in the paper.The first two NOCV pairs cannot combine to form bonding orbitals, which is chemically meaningless.To rule out the bonding nature, the fragmentation method with different charge and electron distribution should be tested by comparing their total value of the ΔEorb term.After selecting reasonable interacting moieties, the bonding nature can be determined.
The author proposed a nitrogen activation model with silicon-H2 distinct from carbene nitrogen fixation.Initially, Si(0) coordinates with nitrogen, followed by oxidative addition of dihydrogen to form a H2Si:N2 species.Subsequently, a series of nitrogen activation processes ensue.This is highly intriguing, and I have no objections to these findings.It is foreseeable that achieving highly reactive free Si(0) species in synthetic chemistry is challenging.An open question arises: Is the H-substituted group in H2Si:N2 crucial for further enhancing nitrogen activation?This could have significant implications for nitrogen activation in synthetic chemistry.
Reviewer #2: Remarks to the Author: Wang, Riedel and their co-workers reported a combined experimental and theoretical study on the dinitrogen activation by silylene.Three species including H2Si(μ-N)2SiH2, H2SiNN(H2) and HNSiNH complexes were characterized by infrared spectroscopy in conjunction with quantum-chemical calculations.The topic is interesting and the findings of this manuscript is significant enough for the publication in this journal.Some issues are listed below for the authors' consideration.
1.The Lewis structure of compound A should be given.Currently, only a circle is drawn in Scheme 1.Its resonance structures should be provided and the dominated one should be specified if possible to help the readers understand the Si-N and N-N bonding because currently it is not clear whether the Si-N is a single or double bond.2. Is the four-membered ring in species A aromatic or not even though it is puckered?The authors only provided the NICS values.Note that many aromaticity descriptors have been proposed on the basis of multiple manifestations of aromaticity.However, one must utilize a set instead of a single one to justify aromaticity in a given system (DOI: 10.1016/j.fmre.2023.04.004).The conclusion based on a single aromaticity index could be wrong.For instance, the recent results (DOI: 10.1021/jacs.3c07335) based on magnetic and electronic descriptors of aromaticity together with 11B{1H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12]2+, behave as doubly 3D-aromatic compounds.However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I12 shell, in contrast to the 10σ-electron I62+ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system.Therefore, the [B12I12]0/2+ species cannot be considered as doubly 3Daromatic.Thus, other indices e.g., EDDB or the energetic one should be used to confirm the findings in this manuscript.3.Even for the NICS calculations, the authors could consider the CMO-NICS to separate the sigma and pi contributions.The last not the least, the NICS index is claimed to be a qualitative rather than quantitative method to evaluate aromaticity.4. The recent study on din (DOI: 10.1002/jcc.27281;10.1021/acs.inorgchem.1c03546;10.1016/j.fmre.2023.04.004) should be discussed and cited. 5. Figure S8 should be moved from SI the manuscript and the Lewis structure of all the computed structures should be drawn.
Reviewer #3: Remarks to the Author: In this manuscript, the authors described the N2 cleavage by the reaction of the parent silylene, H2Si:, at low temperature in dinitrogen matrix resulting in the formation of H2Si(m-N)2 SiH2.Small molecule activation is one of the hot topics in chemical transformation from the viewpoint of synthetic chemistry, and furthermore, the activation of nitrogen molecule with a new method is of particular importance in view of the development of effective supply of ammonia.The present work is very timely to show the intrinsic reactivity of the simplest silylene with nitrogen and the results here obtained will contribute further progress in the chemistry of metallylenes in small molecule activation.The experimental results are well interpreted in terms of the spectroscopic analyses in matrix and the identification of the reaction products is reasonably supported by the results of theoretical calculations.Although the reviewer is essentially satisfied the results and conclusions of this work and recommends this manuscript for the publication in Nature Communications, the following points should be considered and adequate modification should be made before the final acceptance.
1) As for the reaction(s) examined in this manuscript as a subject, some schematic drawings should be added in the initial part of the text (not in the Supporting Information such as Figures S8 and S9).Otherwise, it might be somewhat difficult for the general readers unfamiliar with matrix isolation and main group chemistry.Especially, the silylene formation in the matrix with irradiation should be shown together with the subsequent reaction(s) with a nitrogen molecule.
2) The authors performed precise calculations for the reaction profile of the activation of a nitrogen molecule with H2Si:.However, H2Si: is only accessible in the low-temperature matrices.How about the case of more practical silylenes?Is it possible to show some information (the results of theoretical approach) for the reactions of typical models of substituted silylenes such as Me2Si: or Ph2Si: with a nitrogen molecule?3) In addition, the evaluation of aromaticity of some molecule is very sensitive.Is the evaluation of A using NICS(1)av.most suitable?From the structural determination, the authors concluded A is not a planar but puckered molecule.In such case, it should be more careful to use the NICS evaluation.If the molecule is planar, NICS(1)zz is more reliable.How about other evaluation methods for the aromaticity of A?

Responses
Reviewer 1 Q1: The author proposed a nitrogen activation model with silicon-H 2 distinct from carbene nitrogen fixation.Initially, Si(0) coordinates with nitrogen, followed by oxidative addition of dihydrogen to form a H 2 Si:N 2 species.Subsequently, a series of nitrogen activation processes ensue.This is highly intriguing, and I have no objections to these findings.It is foreseeable that achieving highly reactive free Si(0) species in synthetic chemistry is challenging.An open question arises: Is the H-substituted group in H 2 Si:N 2 crucial for further enhancing nitrogen activation?This could have significant implications for nitrogen activation in synthetic chemistry.
We added "Similar exothermic reactions could occur when H-substituted groups, such as CH 3 and Ph, are applied." in line 195.Q3.Page2 line 37 "A substantially elongated N-N bond has been achieved" which should be described as a crucial metric for nitrogen activation, even though the stretching of nitrogennitrogen bonds is typically considered a key indicator of nitrogen activation.
Answer: This sentence has been revised to "A crucial metric for nitrogen activation of substantially elongated N-N bond has been achieved" at Page2 line 40.

Q4.
Page2 line 41 While boron indeed serves as a classic example of nitrogen activation, carbenes, as elements of Group 14, have matured significantly in terms of nitrogen activation under matrix isolation conditions.Furthermore, carbenes appear to be more relevant to the topic.I recommend that the author cite relevant literature and provide a brief discussion.

Answer:
The relevant literature about carbenes has been cited and a brief discussion has been added just after the introduction of boron at Page2 line 45-49."Carbene, another reactive intermediate, has also been used for N 2 activation and conversion. 17-19Maier et al. found that singlet  0  2 carbene (2-diazo-2H-imidazole) would bind with dinitrogen in the matrix, demonstrating the potential for  0  2 carbene to activate dinitrogen. 20Furthermore N 2 activation by a carbene pair has be calculated and the N≡N triple bond was predicted to be elongated to N-N single bond (1.428 Å) under the synergistic effect of the two CH 2 moieties. 21" Q5.Page2 line 52 The author describes the ability of silylenes to activate a range of small molecules, which indeed contributes to our understanding of their reactivity.Subsequently, nitrogen activation is also mentioned as a possibility.However, the logic in this sentence appears somewhat disjointed.In fact, silylenes do exhibit high reactivity, sharing similarities with certain carbenes that have a narrow HOMO-LUMO energy gap.Nevertheless, the challenge of activating more inert nitrogen by silylenes remains undiscussed.I recommend that the author briefly address the key difficulties in silylene-mediated nitrogen activation to enhance the significance of this work.
Answer: We added "The key difficulty in silylene-mediated nitrogen activation is to modify the occupied and vacant orbitals of silylene in space and energy, which could enhance weakening and functionalization of an inert chemical bond.For example, Driess et al. reported that two silylene moieties (bis-silylenes) could be cooperative to cleaving unreactive bonds, in which the Si---Si distance plays a crucial role. 31,35"     Q6.Page3 line 68：The description 'silylenes: N2 products' appears to be non-standard.

Answer:
The description 'silylenes: N2 products' has been changed to 'adduct products' at Page 4 line 76.
Q7. Page 4 line 73 How does secondary irradiation (at 220 nm) specifically impact the generation of compound A? During A's formation, is it possible for it to proceed via interaction with an H2SiN2 side-on complex and another transient H2Si: molecule, directly cleaving nitrogen, while B predominantly serves as the dissociator of N2 in this process?Indeed, the nitrogen molecules in compound B have not been significantly activated.Furthermore, can the formation of H2Si: be observed under an argon atmosphere without the use of nitrogen?If possible, could further introduction of nitrogen verify the presence of H2Si: as another crucial reactive intermediate in the formation of compound A?
Answer: Yes, the H 2 Si: can be generated in argon matrix for sure (J.Phys.Chem.A 2002, 106, 7696-7702.),but in nitrogen matrix the H 2 Si: was stabilized as H 2 SiNN because of nitrogen circumstance.
Compound A was observed upon the first λ > 220 nm irradiation.The following annealing had no impact to product A and B, but the precursor, but SiNN, can remove and diffuse in matrix and some of SiNN will be ready to interact with B. Further, the second λ > 220 nm irradiation will induce the reaction and give more compound A.
There is no evidence for A's formation via interaction of an H 2 SiN 2 side-on complex with another transient H 2 Si:.First of all, H 2 SiN 2 side-on complex is 21.9 kcal/mol higher in energy than that of end-on complex with B3LYP/6-311++g(3df,3pd). Secondly the significant NN stretching frequency is calculated at 1568.2(157) cm −1 , which was not observed in our experiments.Thirdly when H 2 Si: is generated, which is immediately interacted with N 2 to form H 2 SiNN in N 2 circumstance.What's more, we carried out the Tesla coil discharge reactions of SiH 4 with or without H 2 in excess solid N 2 , both H 2 SiN 2 and species B can be observed while species A is missing due to the lack of .
It is actually that the formation of H 2 Si: can be observed under an argon atmosphere without the use of nitrogen.{J.Phys.Chem.A 2002, 106, 7696-7702.}However, when we tried to introduce to the N 2 , there was no evidence for the formation of compound A (Figure S23) and the absorptions of H 2 Si: showed no significant changes after irradiation.Q8.Page5 line 108: In Figure 1a, I noticed a broad peak around 1950 cm -¹, which may also indicate a N2-complex.Could it be related to the formation of compound B, considering that this signal peak disappears in Figure 1b?Additionally, in Figure 1b, the signal intensity of NNSiNN sharply decreases, and under secondary irradiation (Figure 2e), the NNSiNN signal further diminishes.What is the relationship between NNSiNN, SiNN, H 2 SiNN, and the generation of compounds B and A?" Answer: The broad band centered at 1950 cm -¹ is the baseline without correction.In the isotopic experiments, the infrared spectra of the laser-ablated Si atoms reactions with 10% D 2 in 14 N 2 matrix and with 10% H 2 in 15 N 2 matrix, neither 15 N counterpart around 1885 cm −1 (14N/15N isotopic ratio 1.034) in the laser-ablated Si atoms reactions with 10% H 2 in 15 N 2 matrix nor D counterpart around 1393 cm −1 (H/D isotopic ratio 1.400) in the laser-ablated Si atoms reactions with 10% D 2 in 14 N 2 matrix was found.Thus, the broad band could be the uneven baseline rather than NN nor Si-H stretching modes.
The reaction between SiNN ( 3  -) and dinitrogen to give NNSiNN ( 1 A 1 ) is endothermic by 5.4 kcal/mol calculated with B3LYP/6-311++g(3df,3pd) and NNSiNN will diminish upon irradiation of the matrix with λ = 313 nm (Organometallics 2000, 19, 4775-4783.).To verify the relationship of NNSiNN and SiNN, laser-ablated Si atoms reactions with pure N 2 were done (Figure S18).The absorptions of NNSiNN diminished again and the absorptions of SiNN increased by 10% after λ > 300 nm irradiation.In our experiments of laser-ablated silicon atoms with H 2 in dinitrogen matrix, SiNN ( 3  -) was supposed to be a starting compound to give the complex H 2 SiNN ( 1 A 1 ) and complex B with H 2 upon λ > 300 nm irradiation.It is certainly difficult to find H 2 Si in solid nitrogen, which will react with N 2 in our experiment with no energy barrier.Thus, main product SiNN on codeposition was supposed to be the reactant to give product Answer: The fate of complex B either reacts with SiNN to form product A, or gives SiH 4 through H 2 bond broken under 220 nm irradiation.The complex B can be trapped because of N 2 matrix, otherwise the SiH 2 can react with H 2 to give SiH 4 with very small energy barrier of 1.4 kcal/mol (Can.J. Chem. 2000Chem. , 78, 1428Chem. -1433)).

Q10. Page8 line 173:
The N-N bond length is between 1.8-1.9 A. It should be noted that there is a minor bonding interaction between two nitrogen atoms under this value.From the perspective of dinitrogen activation, in this case, the N-N bond is fully cleaved and each N part should be assigned as N 3-.In the EDA-NOCV analysis, however, the authors have chosen two neutral fragments for comparison.The rationality of this choice should be discussed in the paper.The first two NOCV pairs cannot combine to form bonding orbitals, which is chemically meaningless.To rule out the bonding nature, the fragmentation method with different charge and electron distribution should be tested by comparing their total value of theΔE orb term.After selecting reasonable interacting moieties, the bonding nature can be determined.

Answer:
The choice of the fragments is determined by the question of interest.Trivially, in a diatomic molecule X 2 the two atoms are chosen as interacting fragments, but when a heteroatomic species such as LiF shall be analyzed.The possibility of using different fragments for the bonding analysis provides more flexibility that may be used to address different questions about the bonding nature.Thus, the choice of neutral Li and F as interacting fragments includes all changes along the bond formation between the isolated atoms toward LiF, whereas the choice of the ions Li+ and F− addresses the question about the nature of the eventually formed bond.(Chem. Rev. 2019, 119, 8781−8845).
In our work, the neutral fragments N 2 ( 1 Ʃ g ) and (SiH 2 ) 2 ( 1 A) in the singlet state, which refer to the symmetry-allowed dissociation products, have been selected as interacting moieties which addresses the question about all changes along the bond formation between two neutral fragments (page 9, line 197-199).We redo the EDA-NOCV calculation at the meta-Hybrid/M06-2X/TZP level instead of BP86/TZP level.The results are shown in Table S8.The first two NOCV pairs could combine to form bonding orbitals, which is chemically meaning now (Figure 4).The breakdown of the orbital interaction into pairwise orbital interactions reveals that the dominant orbital stabilization, ΔE orb(1) (-329.1 kcal mol -1 ) and ΔE orb(2) (-318.7 kcal mol -1 ), comes from the back-donation of the HOMO-1 (mainly 3p x of the Si atom) and the HOMO (mainly 3p x of the Si atom) of the (SiH 2 ) 2 moiety into the two perpendicular π* MOs of the N 2 ligands, known as push effect.The most interesting orbital interactions are ΔE orb(3) and ΔE orb(4) , which contribute to the donation of π MO electrons of the N 2 ligands to the LUMO+1 (3p z of Si atom and 1s of H atom) and LUMO (mainly 3s, 3p y , 3p z and 4s of Si atom) of the (SiH 2 ) 2 fragment (pull effect) (page 9, line 199-206).Tables S13 and Figure S25 show the EDA-NOCV results for A using neutral and charged fragments ((N 2 ) 2-, (N 2 ) 4-, and (N 2 ) 6-) as interacting moieties.The smallest ΔE orb values are found when the doubly charged species (SiH 2 ) 2 2+ ( 3 B 2 ) and (N 2 ) 2-( 3 ∑ g ) are used for the calculations, which is a measure for the best description of the bonds finally formed.The EDA-NOCV results in Table S13 suggest the orbital term ΔE orb accounts for 70% of the total attraction between the neutral units, including the polarization within the fragments during bond formation.However, the dominance of covalent bonding disappears when the final bonding situation is analyzed.The electrostatic part of the attractive interactions constitutes greater than 50% of the total attraction.

Table S13. EDA-NOCV results of H
Wang, Riedel and their co-workers reported a combined experimental and theoretical study on the dinitrogen activation by silylene.Three species including H 2 Si(μ-N) 2 SiH 2 , H 2 SiNN(H 2 ) and HNSiNH complexes were characterized by infrared spectroscopy in conjunction with quantumchemical calculations.The topic is interesting and the findings of this manuscript is significant enough for the publication in this journal.Some issues are listed below for the authors' consideration.
Q1.The Lewis structure of compound A should be given.Currently, only a circle is drawn in Scheme 1.Its resonance structures should be provided and the dominated one should be specified if possible to help the readers understand the Si-N and N-N bonding because currently it is not clear whether the Si-N is a single or double bond.Q2.Is the four-membered ring in species A aromatic or not even though it is puckered?The authors only provided the NICS values.Note that many aromaticity descriptors have been proposed on the basis of multiple manifestations of aromaticity.However, one must utilize a set instead of a single one to justify aromaticity in a given system (DOI: 10.1016/j.fmre.2023.04.004).
The conclusion based on a single aromaticity index could be wrong.For instance, the recent results (DOI: 10.1021/jacs.3c07335) based on magnetic and electronic descriptors of aromaticity together with 11B{1H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12]2+, behave as doubly 3D-aromatic compounds.However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I12 shell, in contrast to the 10σ-electron I62+ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system.Therefore, the [B12I12]0/2+ species cannot be considered as doubly 3D-aromatic.Thus, other indices e.g., EDDB or the energetic one should be used to confirm the findings in this manuscript.
Answer: Thank you for your valuable suggestion.The article (DOI: 10.1021/jacs.3c07335)evaluated aromaticity from three aspects: magnetic properties with the nucleus-independent chemical shift (NICS) method, electron delocalization properties with the electron density of delocalized bonds (EDDB G ) function and energy properties with aromatic stabilization energy (ASE) through homodesmotic reactions.In our article, more aspects except for magnetic properties should be taken into account.
Bond critical points (BCP) between both Si-N and N-N are found with atoms-in-molecules (AIM) {Accounts.Chem.Res.1985, 18, 9-15} methodology (Figure S13).The BCP between Si and N atoms locates in the region with positive Laplacian value and the accumulation of electronic charge, suggesting polar covalent interaction.However, a BCP between two nitrogen atoms may be found without a region of electronic-charge accumulation, which means the two atoms may not be bonded to each other.What's more, electron localization function (ELF) {Angew.Chem.Int.
Edit. 1997, 36, 1809-1832} for A also demonstrates that there is no covalent interaction between two N atoms (Figure S14).Thus, two aromatic -electron delocalization in the cyclic compounds A was proposed following the (4n+2) π electrons Hückel rule.For a molecular orbital (MO) analysis at B3LYP/6-311++G(3df,3pd) level with Gaussian 09, HOMO-6 orbital shows typical bonding orbitals (page 5, line 115-120) and the strongest  aromaticity is proved by the following CMO-NICS(1) ZZ analysis.(page 6, line 126-129) For electronic aspects, the multi-center bond order (MCBO), which is also known as multi-center index (MCI) {Struct.Chem.Q5. Figure S8 should be moved from SI the manuscript and the Lewis structure of all the computed structures should be drawn.
Answer: Figure S8 has been moved from SI to the article (Figure 5) and the Lewis structures of products A, B, C were present in scheme 1.

Reviewer 3:
In this manuscript, the authors described the N2 cleavage by the reaction of the parent silylene, H2Si:, at low temperature in dinitrogen matrix resulting in the formation of H 2 Si(μ-N) 2 SiH 2 .Small molecule activation is one of the hot topics in chemical transformation from the viewpoint of synthetic chemistry, and furthermore, the activation of nitrogen molecule with a new method is of particular importance in view of the development of effective supply of ammonia.The present work is very timely to show the intrinsic reactivity of the simplest silylene with nitrogen and the results here obtained will contribute further progress in the chemistry of metallylenes in small molecule activation.The experimental results are well interpreted in terms of the spectroscopic analyses in matrix and the identification of the reaction products is reasonably supported by the results of theoretical calculations.Although the reviewer is essentially satisfied the results and conclusions of this work and recommends this manuscript for the publication in Nature Communications, the following points should be considered and adequate modification should be made before the final acceptance.

Q1.
As for the reaction(s) examined in this manuscript as a subject, some schematic drawings should be added in the initial part of the text (not in the Supporting Information such as Figures S8  and S9).Otherwise, it might be somewhat difficult for the general readers unfamiliar with matrix isolation and main group chemistry.Especially, the silylene formation in the matrix with irradiation should be shown together with the subsequent reaction(s) with a nitrogen molecule.
Answer: Thank you.Figures S8 and S9 has been moved from SI to the article (Figures 5 -6) to intuitively give a better understanding of the formation of product A and B (Figure 5), and the formation of product C (Figure 6).

Q2.
The authors performed precise calculations for the reaction profile of the activation of a nitrogen molecule with H2Si:.However, H2Si: is only accessible in the low-temperature matrices.How about the case of more practical silylenes?Is it possible to show some information (the results of theoretical approach) for the reactions of typical models of substituted silylenes such as Me2Si: or Ph2Si: with a nitrogen molecule?
Q3.In addition, the evaluation of aromaticity of some molecule is very sensitive.Is the evaluation of A using NICS(1)av.most suitable?From the structural determination, the authors concluded A is not a planar but puckered molecule.In such case, it should be more careful to use the NICS evaluation.If the molecule is planar, NICS(1)zz is more reliable.How about other evaluation methods for the aromaticity of A? Answer: While the NICS index was originally obtained for planar aromatic systems, it has recently been suggested to calculate an average NICS(1)av index, NICS(1)av = [NICS(−1) + NICS(1)]/2, as a probe of aromaticity in nonplanar molecular systems.{RSCAdv.2016, 6, 23900-23904}.We also calculated the anisotropy of the induced current density (AICD) but the result is not clear.
Except for the magnetic properties with the nucleus-independent chemical shift (NICS) method, we evaluate the aromaticity of A from mainly two aspects.First of all, two aromatic -electron delocalization in the cyclic compounds A was proposed following the (4n+2) π electrons Hückel rule.A BCP between two nitrogen atoms may be found without a region of electronic-charge accumulation in the atoms-in-molecules (AIM) {Accounts.Chem.Res.1985, 18, 9-15} methodology (Figure S13), which means the two atoms may not be bonded to each other.Electron localization function (ELF) {Angew.Chem.Int.Edit.1997, 36, 1809-1832} methodology find there is no covalent interaction between two nitrogen atoms (Figure S14) but two aromatic electron delocalization in the cyclic compounds A which is proved by a molecular orbital (MO) analysis (Figure S27) and computed canonical molecular orbital natural chemical shielding (CMO-NICS(1) ZZ ) values (Figure S28).HOMO-6 orbital shows typical -bonding orbitals (Figure S27) and makes the largest diatropic contribution of -6.36 ppm for the  aromaticity.(Figure S28) What's more, the multi-center bond order (MCBO), which is also known as multi-center index (MCI) to evaluate aromaticity from the aspect of electron delocalization properties {Struct.Chem. 1990, 1, 423-427;Phys. Chem. Chem. Phys. 2016, 18 (17)     1.The authors carried out additional NRT calculations to determine the Lewis structure of product H2Si(-N)2SiH2 (Figure S26).Actually, it is quite reasonable to me.However, the authors mentioned that "However, our following atoms-in-molecules (AIM) and electron localization function (ELF) methodology find there is no covalent interaction between two nitrogen atoms but two aromatic -electron delocalization in the cyclic compounds A with a molecular orbital (MO) analysis".So how can the reader understand such a contradiction?Even the authors prefer an aromatic structure in the latter, the dominated Lewis structures (one or two) should be also provided to help the reader understand the structure.For instance, even for the aromatic benzene ring, two Lewis structures can be drawn.2. The CMO-NICS calculations were also carried out.Again, it is problematic.As shown in Figure S28, three pi-MOs are present and the authors mentioned HOMO-6() orbital making the largest diatropic contribution of -6.36 ppm for the  aromaticity.(page 6, line 126-129).If that is the case, is it a two or six- electrons in this species?How can two nitrogen atoms contribute two or six- elctrons?3. The authors mentioned that the formation of R2SiNN is slightly exothermic by 6.2 kcal/mol (H2SiNN), indicating the unsubstituted one can produce the lowest reaction energy, which is similar to the previous finding on dinitrogen activation (DOI: 10.1016/j.cclet.2022.107759)and thus should be discussed and compared.4. The page number of Ref.12 is not available.Thus, the DOI number rather than the page number should be used.5.The computed activation energies in Figures 5 and 6 are both particularly high, is it in line with the experimental conditions?
Reviewer #3: Remarks to the Author: In the revised version,the authors made adequate modification according to the comments of reviewers.They added necessary information based on further experimental and theoretical results, which are appropriately appeared in the main text and supporting information.The reviewer is now satisfied with the responses to the referees' comments and the revised manuscript.

Responses
Reviewer 2: Wang, Riedel, Xu and their co-workers addressed most concerns in the first round of the review.Still, some issued should be solved before the publication of this manuscript.
Q1.The authors carried out additional NRT calculations to determine the Lewis structure of product H 2 Si(-N) 2 SiH 2 (Figure S26).Actually, it is quite reasonable to me.However, the authors mentioned that "However, our following atomsin-molecules (AIM) and electron localization function (ELF) methodology find there is no covalent interaction between two nitrogen atoms but two aromatic π -electron delocalization in the cyclic compounds A with a molecular orbital (MO) analysis".So how can the reader understand such a contradiction?Even the authors prefer an aromatic structure in the latter, the dominated Lewis structures (one or two) should be also provided to help the reader understand the structure.For instance, even for the aromatic benzene ring, two Lewis structures can be drawn.Q2.The CMO-NICS calculations were also carried out.Again, it is problematic.As shown in Figure S28, three pi-MOs are present and the authors mentioned HOMO-6(π) orbital making the largest diatropic contribution of -6.36 ppm for the π aromaticity.(page 6, lines 126-129).If that is the case, is it a two or six-π electrons in this species?How can two nitrogen atoms contribute two or six-π elctrons?
Answer: To quantify delocalized electrons within the complex A, the electron density of delocalized bonds (EDDB) analysis was used and 1.55 electrons delocalization was calculated (Table S14) in which similar to L 2 Si 2 P 2 (L = PhC(NtBu) 2 ) where 1.77 electrons were found delocalization over the whole Si 2 P 2 skeleton confirmed by the electron localizability index (ELI-d) and a natural bond orbital (NBO) analysis (Angew.Chem.Int. Ed. 50, 2322-2325(2011).)and the tetraatomic boron specie where 1.57 electrons were found delocalization and  electrons dominated in the B 4 framework (Organometallics 39, 2602-2608 (2020).).In our system, aromaticity is also the dominant one (60 %) in line with the CMO-NICS calculations.The EDDB analysis has been added on Page 5 Lines 108-109 and 120-124.'Canonical molecular orbital natural chemical shielding (CMO-NICS(1) ZZ ) was calculated at B3LYP/ 6-311++G(3df,3pd) level to separate the  and  contributions of canonical molecular orbital, and larger diatropic contribution of -14.6 ppm from  orbitals compared with -11.9 ppm from  orbitals (Figure S28) indicates σ aromaticity dominated in A, consistent with the EDDB analysis where 1.55 electrons delocalization was calculated (Table S14) which are comparable to the number of delocalized electrons in L 2 Si 2 P 2 (L = PhC(NtBu) 2 , 1.77 electrons) (Angew.Chem.Int.
Ed. 50, 2322-2325 (2011).)and the tetraatomic boron specie (1.57electrons) (Organometallics 39, 2602-2608 (2020).)and  aromaticity is also the dominant one (60 %).' What's more, the gauge including magnetically induced current (GIMIC) method is applied to obtain magnetically induced current densities in molecules and a net diamagnetic ring current can demonstrate the aromaticity (J.Chem.Phys. 121, 3952-3963 (2004); Phys.Chem. Chem. Phys. 13, 20500-20518 (2011).).The diatropic and paratropic ring currents for complex A are 29.6 nA T -1 and -18.2 nA T -1 , respectively, yielding a net ring-current strength of 11.4 nA T -1 calculated at B3LYP/6-311++g (3df, 3pd) level similar to the typical aromatic molecule benzene (11.8 nA T -1 ) (J. Phys.Chem.A 113 (2009).)which means that complex A is an aromatic molecule (Figure S27).The GIMIC analysis has been added on Page 5 Lines 107-108 and 112-114.'GIMIC method was calculated at B3LYP/6-311++g (3df, 3pd) level and a net diamagnetic ring current of 11.4 nA T -1 in A similar to the typical aromatic molecule benzene (11.8 nA T -1 ) (J. Phys.Chem.A 113, 8668-8676 ( 2009)) could demonstrate the aromaticity of complex A (Figure S27).'  Answer: The computed activation energies are actually in line with the experimental conditions since the reaction barrier can be overcome by the energy provided by photolysis and laser ablation in our experiments.In practice, there are at least five ways for atoms to be activated and thereby induced to react in matrix-isolation experiments, namely, by (i) photolysis, (ii) gaseous discharge, (iii) laser ablation, (iv) the chemical reaction giving rise to them, and (v) thermal means.(Chem. Rev. 102, 4191-4241 (2002); Russ.Chem. Rev. 90, 1142-1165(2021).).Thus, in our experiments, both highly energetic laser-ablated silicon atoms and broad-band UV light ranging from 300 to 220 nm can provide energy to overcome the energy barrier.Photolysis is the most effective and controlled way to induce an atomic reaction and the nature of the excitation can be appraised with a well-defined wavelength.For example, neglecting energy loss, the energy provided by a Hg arc that emits broad-band UV-vis light (> 220 nm) is 5.6 eV (130 kcal mol -1 ).When considering the energy loss, an activation energy of 229 kJ mol -1 (55 kcal mol -1 ) is also well within the reach of near-UV radiation (240<<380 nm).(J.Phys.Chem.A 115, 8638-8642 (2011).) Upon  > 300 nm irradiation, the complex H 2 SiNN was observed (Figure 1b) with the activation energy (from SiNN and H 2 ) much less than 19.3 kcal mol -1 (Figure 5).Afterward, H 2 SiNN reacts with a second H 2 molecule to form complex B. The corresponding barrier was computed to be only 1.8 kcal mol -1 at DFT level which is in line with the increase of H 2 SiNN and complex B and the decrease of SiNN upon  > 300 nm irradiation in the experiments.
What's more, complex A observed upon  > 220 nm irradiation with the decrease of complex B and SiNN is in line with the highest activation barrier computed to be 38.4 kcal mol -1 .Although the activation barrier of Complex C is computed to be 32.8 kcal mol -1 , C is observed in freshly deposited samples and it increased markedly after  > 220 nm irradiation, in which the laserablated energy provided in the codeposition process and the irradiation energy at  > 220 nm most likely support the formation separately.Similarly, the barrier from SPN to cyc-PSN calculated to be 138 kJ mol -1 (33 kcal mol -1 ) can be overcome by UV light irradiation (=255 nm) (Angew.Chem.Int. Ed. 51, 3334-3339 (2012).).
Reviewers' Comments: Reviewer #2: Remarks to the Author: Wang, Riedel, Xu and their co-workers addressed most concerns in the second round of the review.Now, only one issue should be solved before the publication of this manuscript.That is the contradiction between the aromaticity and the Lewis structure.The authors show in Scheme 1 two lone pairs on the nitrogen atoms in the dominate resonance.Even all these lone pairs can be used to contribute to the aromaticity, it is only 4pi electrons.How can a species with 4pi electrons be aromatic?In addition, as the authors show a N-N single bond of A in Scheme 1.Is such a single bond supported by the bond order calculations?What are the Wiberg bond index or Fussy bond order of such a N-N bond in A?

Responses
Reviewer 2: Wang, Riedel, Xu and their co-workers addressed most concerns in the second round of the review.Now, only one issue should be solved before the publication of this manuscript.That is the contradiction between the aromaticity and the Lewis structure.The authors show in Scheme 1 two lone pairs on the nitrogen atoms in the dominate resonance.Even all these lone pairs can be used to contribute to the aromaticity, it is only 4pi electrons.How can a species with 4pi electrons be aromatic?
In addition, as the authors show a N-N single bond of A in Scheme 1.Is such a single bond supported by the bond order calculations?What are the Wiberg bond index or Fussy bond order of such a N-N bond in A?
Answer: The two lone pairs of the nitrogen atoms are localized and don't exhibit delocalization characteristics in this species.Thus, the four lone electrons on the nitrogen atoms can't contribute to the aromaticity.For clarity, two lone pairs on the nitrogen atoms in the dominate resonance have been removed in Scheme 1 on Page 21(now Figure 1).
The Mayer bond order between N-N atoms in A is 0.689 calculated at B3LYP/6-311G(3df,3pd) level.In addition, Fussy bond order of 0.785 and Wiberg bond order of 1.088 are also obtained.The analysis for bond order is added on Page 5 Lines 104-105.'A Mayer bond order of the N-N bond for molecule A is 0.689 computed at the B3LYP/6-311G(3df,3pd) level.'

Figure R1 .
Figure R1.Potential energy surface for the reaction of H 2 Si + N 2 + SiH 2 → A computed at the B3LYP/6-311++G(3df, 3pd) level of theory.The unit of relative energy is kcal/mol.

Figure 4 .
Figure 4. Shape of the deformation densities, Δρ (1)-(4) of H 2 Si(-N) 2 SiH 2 corresponding to ΔE orb(1) -ΔE orb(4) and the associated fragment orbitals at the meta-Hybrid/M06-2X/TZP level.Isosurface values are 0.004 au.The eigenvalues |v n | give the size of the charge migration in e.The direction of the charge flow of the deformation densities is red→green.

Figure S25 .
Figure S25.Using (SiH 2 ) 2 2-and N 2 2-as interacting fragment and the shape of the deformation densities, Δρ (1)-(4) of H 2 Si(-N) 2 SiH 2 corresponding to ΔE orb(1) -ΔE orb(4) and the associated fragment orbitals at the meta-Hybrid/M06-2X/TZP level.Isosurface values are 0.004 au.The eigenvalues |v n | give the size of the charge migration in e.The direction of the charge flow of the deformation densities is red→green.

Figure S13 .
Figure S13.Laplacian distribution of the charge density of electron localization function (ELF) for H 2 Si(-N) 2 SiH 2 in the plane Si-N-N.

Figure 1 .
Figure 1.Binding modes of small molecules to silylene.(Dipp = 2,6-i Pr 2 C 6 H 3 ; TBoN = Even for the NICS calculations, the authors could consider the CMO-NICS to separate the sigma and pi contributions.The last not the least, the NICS index is claimed to be a qualitative rather than quantitative method to evaluate aromaticity. Edit. 2021, 60 (31), 17205-17210.}The analysis is added in the article at Page 5 Line 120-123.Q3.Answer: The literature about dinitrogen activation by five-electron boron-centered radicals and five-electron boron-centered radicals (DOI: 10.1002/jcc.27281;10.1021/acs.inorgchem.1c03546) has been discussed and cited at Page 2 Line 38-40.The review about the application of aromaticity and antiaromaticity (DOI: 10.1016/j.fmre.2023.04.004) has been discussed and cited at Page 5 Line 116.
Answer: Thank you for pointing this out.The page number of Ref.12 is now updated and available on its website and Ref.12 has been changed to 'Zeng, J., You, F. Y. & Zhu, J.